The circuit techniques which have generally been employed for controlling such a servo loop have included the use of a pair of switching circuits controlled by the early and late gate pulses arranged to feed type of capacitive charge network (sample and hold). When the early gate switch turns on, the capacitive network charges to a level determined by the video signal then present. A second such arrangement is controlled by the late gate. The signal level stored in the two sample and hold networks following termination of the late gate are compared and the difference is utilized as an error signal to drive the servo loop to readjust the timing of the gates.
A difficulty with this arrangement is that the capacitive networks used to sample and hold the video signal tend to be "end reading". This means that the charge level stored on the capacitor essentially represents the level of the video signal at the instant that the early or late gate pulse terminated. The servo system, in seeking to equalize the levels stored on the early and late gate capacitors, tends to center the late gate sampling pulse, rather than its leading edge, on the target pulse. If the time constants of the capacitive sampling networks are increased in an effort to impart integrating characteristics to the networks, the voltage levels reached by the capacitors during the sampling interval become very small and the consequent accuracy requirements for the other circuits in the loop becomes much greater. This tends to make the system more complex and expensive.
An alternate technique that is sometimes employed includes the use of a pair of matched, complementary video channels. These may be sampled by early and late gate controlled switches and the outputs therefrom fed to an integrating amplifer. The principal difficulty with this arrangement is that it is very difficult to obtain precisely matched complementary video channels without utilizing rather involved and expense circuit components.